Vertebrates use tryptophan in protein synthesis and as a precursor of seretonin, kynurenine, quinolinic acid. Ultimately, the levels of these metabolites of tryptophan are determined by tryptophan availability and the levels and efficiencies of the enzymes competing for substrate. Tryptophan oxygenase (TO) and indoleamine dioxygenase (IDO) are two enzymes that metabolize tryptophan via the kynurenine pathway. Experiments were performed to localize the mRNAs for these enzymes in animal tissues and cell culture, and to examine regulation of their mRNA levels. Using RNA-PCR, mRNAs for both enzymes were identified in mouse brain. The mRNA was also identified in mouse liver, as expected. A human cervical carcinoma cell line, ME180, was shown to increase its levels of IDO mRNA in response to treatment with gamma interferon.